Why does my dog pee when he's excited?

You said it - he pees when he gets exceptionally excited and this is also way of showing he is submissive, intimidated, or to show he is not a threat. It sounds like for your dog, he is just showing excitement. My family dog does this especially when we have guests over. The way to stop this is reduce the excitement level when things normally induce this behavior. For example, when you get home or someone comes over, don't overexcite your dog by saying their name in a high-pitched voice or moving quickly. They do feed off of us, so if you're excited to see someone who is over, they will mirror that. You can also take them outside immediately so they can urinate there and maybe run around to let some of that excited energy out. If your dog is a bit more advanced, you can use some commands like sit, go to your place, down. Good luck!